The present invention relates to a method and apparatus for radio frequency communications, and in particular to a method and apparatus for selecting a spot beam for communication with a radio frequency transceiver in a satellite communications system in which a satellite projects a plurality of overlapping spot beams over an area of the earth""s surface. Each spot beam carries a plurality of communication channels, which are re-used in spot beams which do not overlap. In this way, the communications traffic capacity of a satellite communication system is greatly increased over satellite systems which only use a single beam per satellite.
However, a problem associated with satellites having multiple spot beams is that, in order to communicate with a transceiver, it is necessary to know in which spot beam the transceiver is located, so that an appropriate communications channel can beassigned to the transceiver in that spot beam. This problem arises both for geostationary satellites, because the transceivers may move between and during calls, and for non-geostationary satellites, because the satellites move between and during calls as well.
One solution to this problem is described in the document WO 93/09613, in which each spot beam carries a xe2x80x9cpilot tonexe2x80x9d which identifies that beam. A terrestrial terminal must scan through all of the different channels on which pilot tones are transmitted, and send a signal back to the satellite when a pilot tone is received. However, the requirement to scan a large number of channels leads to delay, and requires complicated circuitry and high power consumption in the terrestrial terminal. Moreover, the pilot tones occupy channels which could otherwise be used for communication and are wasteful of the frequency spectrum.
The document GB-A-2 275 588 discloses a similar system, in which identification information is transmitted in each spot beam. A mobile terminal receives the information and transmits the information back via the satellite to a terrestrial network, which registers the location information. To avoid interference between spot beams, the identification information must be transmitted in different channels in each spot beam. This system therefore incurs a similar problem to that in the system described in WO 93/09613.
According to one aspect of the present invention there is provided a method of selecting an appropriate one of a number of spot beams projected by satellite for communication with a terrestrial terminal, in which the spot beams share a return signalling channel. The terrestrial terminal sends a signal in this signalling channel, and the location of the terrestrial terminal is determined by comparing the signal as received in each of the spot beams.
In another aspect of the present invention, there is provided a satellite which has a multi-beam antenna for communication with a terrestrial terminal. The satellite is arranged so that a signalling channel can be received in all the spot beams constructed by the multi beam antenna. The satellite maps the signalling channel received in each spot beam generated by the multi-beam antenna to a plurality of channels in the feeder link to a base station, each of these channels in the feeder link corresponding to the signalling channel received in one of the spot beams.
An advantage of the above aspect of the present invention is that the terrestrial terminal does not need to search a wide range of frequencies, but need only send a signal in the signalling channel. The location of the terrestrial terminal may then be determined at the base station and an appropriate communications channel may be allocated. This method simplifies the operation of the terrestrial terminal and reduces its power consumption. Moreover, since only one signalling channel is used for all the spot beams, more channels can be allocated for communications.
According to another aspect of the present invention, there is provided a method of selecting one of a number or spot beams projected by a satellite for communication with a terrestrial terminal, in which at least three return signalling channels are allocated to the spot beams in such a way that the same signalling channel is not allocated to overlapping spot beams. A terrestrial terminal transmits a signal in each of the signalling channels, and the signal received in each of the signalling channels is compared to determine in which spot beam the terrestrial terminal is located.
Although this method requires that the terrestrial terminal transmit in three signalling channels, it can be advantageously applied to existing satellites which cannot be arranged to assign the same channel to adjacent spot beams.
The average time taken for the terrestrial terminal to indicate its position may be reduced by transmitting a signal to the terrestrial terminal to prevent it from further signalling as soon as the location of the terrestrial terminal is identified. Thus, if the terrestrial terminal is located in a spot beam to which the first signalling channel is assigned, the terrestrial terminal need not proceed to signal in the second and third signalling channels.
Preferably, the satellite is able to project a wide beam which substantially covers the area of all the spot beams, and information is broadcast in the wide beam which identifies which signalling channels may be used. The terrestrial terminals receive this information and transmit signals on the signalling channel or channels indicated by this information. In this way, the signalling channels may be flexibly assigned, and calls may be placed to the terrestrial terminals by broadcasting a call request in the wide beam. Moreover, the terrestrial terminals may be used to communicate via different satellites to which different signalling channels are assigned.
Once the location of the terrestrial terminal is determined, a communications channel may be assigned to the terrestrial terminal within an appropriate spot beam. The appropriate spot beam may be the spot beam in which the strongest signal in the signalling channel or channels was received. Alternatively, where the signal was received in more than one soot beam, a communications channel may be allocated to the terrestrial terminal according to the level of existing communications traffic in each of the spot beams in which the signal was detected. Alternatively, where the satellite is a non-geostationary satellite, a communication channel may be assigned to a spot beam which is approaching the terrestrial terminal.
According to another aspect of the invention, there is provided a method of selecting one of a plurality of cells for communication with a radio frequency transceiver in a cellular communications system, in which signalling channel information is is broadcast over the area of the plurality of cells, the transceiver transmits a signal in a signalling channel indicated by the signalling channel information and one of the plurality of cells is selected for communication with the radio frequency transceiver on the basis of a quality of the signal received in the signalling channel.
The present invention also extends to apparatus for carrying out any of the methods described above.